In a conventional travel assembly to be mounted on a dump truck, a spindle is inserted on a side of an inner periphery of a wheel on which a tire is to be mounted. Wheel bearings are arranged between an outer peripheral wall of the spindle and an inner peripheral wall of the wheel, whereby rotation of the wheel about the spindle is feasible.
A motor is located on a side of an axial end of the spindle. The shaft is connected to an output shaft of the motor such that the shaft can rotate integrally with the output shaft. This shaft is inserted on a side of an inner periphery of the spindle, and extends out from an axially opposite end of the spindle. A reduction gear unit is located on a side of the opposite end of the spindle. The shaft is connected to the reduction gear unit such that an output of the motor can be transmitted. An output side of the reduction gear unit is connected to the wheel such that an output can be transmitted. As a consequence, the output of the motor is transmitted in the order of the shaft, reduction gear unit and wheel so that the wheel rotates together with the wheel.
On a revolving shaft, a centrifugal force acts because of an offset of its center of gravity from its axis due to variations or the like in its manufacture. A revolution speed per unit time at which the shaft is broken by this centrifugal force is called “a critical revolution”. As a technology that sets the critical revolution sufficiently higher than revolution speeds used in a travel assembly, in other words, as a technology that protects a shaft from breakage under centrifugal forces which occur corresponding to revolution speeds to be used, it has been conventionally adopted to interpose a support bearing between an opposite end portion (an end portion on the side of a reduction gear unit) of a spindle and a shaft such that the shaft is supported at a position between a motor and the reduction gear unit. The support bearing is fitted in a retainer secured on the opposite end portion of the spindle (see Patent Document 1).
As another technology for the prevention of breakage of a shaft under centrifugal forces, a shaft is formed hollow to reduce its weight, and hence to lower centrifugal forces (see Patent Document 2)